1. Field of the Invention
The present invention generally relates to a wire grid polarizer and a method for producing the same. More specifically, the invention relates to a wire grid polarizer which is used for optical communication, optical recording, display or the like, and a method for producing the same.
2. Description of the Prior Art
As an example of a polarizer, there is widely known a polarizer utilizing optical anisotropy which is obtained by extending or orienting a high polymer film (see, e.g., Japanese Patent Laid-Open No. 2003-43257). Such a polarizer of a high polymer film is mainly used for a display, such as a liquid crystal panel. However, there are problems in that the heat resistance, moisture resistance and chemical resistance of the polarizer disclosed in Japanese Patent Laid-Open No. 2003-43257 are low, and that the light transmittance and quenching ratio as optical characteristics thereof are also low, although there is an advantage in that it can be produced in large quantities at low costs, since it is produced by using a high polymer film.
As an example of a polarizer having excellent heat resistance, moisture resistance and chemical resistance, there is known a polarizer using a polar core (produced by Coning, Co., Ltd.). The polar core is formed by arranging silver particles in one direction in a glass. The polar core is heated and extended or oriented so as to have polarization characteristics after a metal compound is dispersed in a glass, and it can be used as a polarizer. Such a polarizer has more excellent optical characteristics than those of organic polarizers using high polymer films. There are also known a copper containing polarizing glass which utilizes copper in place of silver, and a method for producing the same (see, e.g., Japanese Patent Laid-Open No. 5-208844). However, it is not easy to produce the polarizer using the polar core and the polarizer disclosed in Japanese Patent Laid-Open No. 5-208844. That is, after a metal compound is dispersed in a glass, it is required to reduce the metal compound, and it is required to heat and extend or orient the glass in one direction. In this case, since it is required to carry out reduction and deposition in a specific gas at a high temperature, there is a problem in that it is required to use a special equipment for carrying out such processes, so that costs are increased. In addition, it is required to carry out heating and extension or orientation, so that it is difficult to produce a large-area polarizer at a time.
By the way, there is known a wire grid polarizer wherein a large number of metal fine wires are formed on a transparent substrate so as to extend in parallel to each other (see, e.g., Japanese Patent Laid-Open No. 10-153706). This wire grid polarizer is produced by applying a resist on a transparent substrate, patterning the resist by the electron beams (EB) lithography or X-ray lithography, and leaving metal fine wires on the transparent substrate by the lift-off method.
However, in a method for producing the wire grid polarizer disclosed in Japanese Patent Laid-Open No. 10-153706, the electron beam (EB) lithography system is unsuitable for the writing in a wide area, since the writing area at a time is small and since it takes a lot of writing time. The X-ray lithography is not a desired method in view of mass productivity and production costs in the present circumstances, since the X-ray lithography system is very expensive and uses a very expensive photomask. In addition, in the method for producing the wire grid polarizer, part of metal pieces lifted off are residual to cause the deterioration of characteristics, and if an expensive noble metal is used, most of metal portions lifted off are useless materials, so that costs are increased.
There are many reports with respect to polarizers utilizing metal wire grids. As shown in FIG. 25, a wire grid polarizer 100 basically has a structure that a large number of metal wires 102 extending in parallel to each other are arranged on a transparent substrate 101 at intervals. In the wire grid polarizer 100 shown in FIG. 25, the metal wires 102 are so set as to have a predetermined period Λ, a predetermined width d and a predetermined thickness t.
When the wire grid polarizer 100 transmits p waves (electromagnetic radiation polarized in parallel to a grid) and damps s waves (electromagnetic radiation polarized in a direction perpendicular to the grid) in transmission characteristics, it is ideal to realize a polarizer having a higher transmittance ratio (Tp/Ts) a smaller loss in transmittance of p wave polarized light on the transmission side, a high quenching ratio and a small loss in a wide band. Thus, it is required to prevent higher-order diffracted light beams from being produced, and it is required to make the wire period Λ half wavelength or less. Moreover, in order to realize a high quenching ratio in a wide band wavelength, it is known that metal fine wires must be produced at a period about one-fifth of wavelength. That is, it is known that this polarizer can be optically regarded as a diffraction grating for sub-wavelength, and that optical characteristics can be very precisely predicted by the strict wave coupling method (RCWA).
FIGS. 26 and 27 show the results of characteristics predicted by the RCWA, i.e., the transmittance and quenching ratio of each of p and s waves in a wavelength band of 50 to 800 nm. As conditions in this case, there are metal wires of gold (n=0.0033, k=4.71), a glass substrate BK-7 (n=1.41), Λ=100 nm, d=40 nm and t=170 nm. As shown in FIG. 26, although the quenching ratio decreases as the wavelength decreases, a good value of 29 dB or more is also ensured at 500 nm (0.5 μm).
However, it is difficult to work a very fine polarizer required as the above described polarizer. First, the wire grid must be formed at a period of about 90 nm which is λ/5 in a wavelength band of visible light of 450 to 650 nm. It is also known that, if the period and width are smaller than λ/5, the quenching ratio can be further improved to realize a high quenching ratio in a wide band. Conventionally, in order to form such a fine pattern, ultraviolet and a photomask are usually used for forming a fine pattern on a photoresist. Until now, a pattern having a size of about 230 nm can be formed at the smallest even if an i-line stepper is used. In recent years, there are exposure systems using eximer laser light, such as ArF or KrF, which has a shorter wavelength. However, there is a problem in that these systems are very expensive in comparison with the i-line stepper which is a conventional exposure system.
As a method expected to be capable of solving such problems of the conventional exposure systems, there is the nano-imprint lithography method. However, even if the nano-imprint lithography method is used, a problem remains when metal wires having a size of 100 nm or less are precisely and repeatably formed in a large area.